The advent of wireless communications at C-band frequencies has fueled a demand for low cost high performance integrated circuits. GHz radio front ends are designed in order to meet the needs of the third generation wireless access systems in terms of bit rate and implementation constraints (size, power consumption, and cost). For this purpose, fully integrated transceivers are a common interest nowadays. Such a transceiver chip is generally coupled to a local oscillator (LO) which provides a clock for data transmission and reception. Generally, the oscillator is thereby connected in a phase-locked loop (PLL) circuit having a phase-locked loop with frequency control means to confine the output phase and frequency of the oscillator within acceptable boundaries.
Often the oscillator is formed out of a number of distributed microwave components, for instance micro strip lines, which are designed to acquire the desired electronic behaviour and characteristics. Depending on their shape and dimensions such distributed components may act either as an inductor, capacitor, resistor or conductor for the supplied signal and they are formed to render the appropriate function within the circuit. These components, however, are difficult to be integrated together with a radio-frequency (RF) system or a PLL-circuit, due to their dimension at microwave frequency. Hence, these components will usually require additional packages and external interconnections. Moreover, these distributed components are generally poorly tunable, as their electrical characteristics strongly depend on the signal frequency.
European patent application 689.287 discloses the basis setup of a more integrated approach of a phase-locked loop circuit with a voltage-controlled oscillator connected to a phase-locked loop. The PLL-circuit comprises frequency control means in the form of a phase detector which is coupled to the output of the voltage-controlled oscillator, while being fed by a reference signal. The differential signal of the phase detector is fed back to the voltage-controlled oscillator via a charge pump and a loop filter to correct any deviation from the intended output frequency. A voltage-controlled oscillator, as used in this prior art circuit, has the advantage that the output frequency can be adjusted to certain extent by means of the voltage supplied to the oscillator, which renders the device suitable for different operating speeds according to different industrial standards.
Although voltage-controlled oscillator can be designed to operate at very high speed, the phase-locked loops which are commercially available mostly cannot handle the high oscillator frequencies required in the next generation wireless communication systems of well beyond 20 GHz. A known solution to this problem is down converting the fundamental oscillator frequency to a level the phase-locked loop can cope with. Another approach is to operate the voltage-controlled oscillator at a lever suitable for the phase-locked loop and up-converting the oscillator frequency to a level required by the RF system according to industrial standards. Both solutions, however, require additional circuit complexity, more packages and more circuit components, e.g. a frequency divider or frequency multiplier, and hence extra chip area, more power consumption together with vulnerable interconnections. Moreover more design uncertainty and frequency noise will be introduced by these additional circuits.
It is an object of the present invention to provide a phase-locked loop and a voltage-controlled oscillator of the kind referred to in the opening paragraph which meet the above drawbacks at least to a significant extent.